248 



Traiisactions of the Society. 



On an average the temperature of the water running to waste 

 was only 1 J° F. higher than when it left the tap. 



Various things may serve to demonstrate the currents, for 

 example small larvee or fine suspended particles. Perhaps the 

 best is a strong solution of some colouring matter, e.g. fuchsin 

 dissolved in salt water. If a drop or two of such a solution is 

 brought gently from a pipette close to the U-tube, one can watch 

 the colouring matter being drawn slowly downwards in a long 

 streak following the tube and then leaving the tube at the middle 

 of the bend to strike rapidly straight down to the bottom. The 

 method will also show the gradual spreading out of the tinctured 

 water over the bottom of the vessel, and the tendency to slow 

 upward return on the inner side of the vessel walls. Again, the 

 concentrated stain introduced gently into any part of the aquarium 

 will be found, within a comparatively short time, to have become 

 evenly distributed throughout the water, while the same stain 

 thus introduced into a similar aquarium without the convection 

 arrangement may take a day or more to reach an equal state of 

 diffusion. 



From a practical point of view the following data regarding 

 the effect of these currents in moving small floating larvae may be 

 of interest. The larvse referred to were healthy swarming starfish 

 gastrulae. The time taken to carry down larvae from near the 

 surface close to the U-tube was approximately 24 seconds for the 

 first 2 in. The next 2 in. were done in 18 seconds, the next 2 in 

 12, the next 2 in about 7, while the last inch of the downward 

 course was hurried over in little more than 2 seconds. The total 

 time from top to bottom was thus something like 63 seconds. At 

 the top the current is slow and indefinite ; lower down it becomes 

 quicker and more restricted in area, while near the bottom it is a 

 narrow and relatively rapid central stream. 



It will readily be understood that the currents are unable in 

 the end to keep debris or dead larvse, or larvae that have lost 

 vitality, from settling on the bottom. Although these objects 

 might all be set a-going at places where the currents are strongest, 

 still they soon find opportunity of sinking down in intermediate 

 areas where the currents are too weak to keep them on the move. 



